This proposal will utilize molecular, genetic and biochemical approaches to decipher the complex interactions between the virion and cell surface molecules that functio in entry, and interspecies spread of RNA viruses. We have developed an in vitro model of virus interspecies transfer that may be reflective of conditions present in xenograph transplant recipients. This model will not only permit the identification of viral and cellular genes which regulate host rante specificity, but also reveal whether phylogenetic homologues of the normal receptor function as natural conduits for interspecies traffic of viruses. We have also developed a model of persistence which characterized by the emergence of variant viruses that display expanded receptor usage in vitro. Using these two complementary models, we will: (1) identify sites of virus-reeptor interaction which regulate Mouse hepatitis Virus (MHV) entry into susceptible cells. Specifically, we will: (1a) characterize host cell phenotypes and virus variants that evolve during MHV persistence, (1b) map sequences in MH?V viriants that recognize alternative receptors for entry into cells, (1c) map recptor residues (Bgp1a, Bgp1b, Bgp2, bCEA) that bind variant viruses, (1d) measure variant virus binding coefficients for soluble receptors, and (1e) determine if in vitro receptor usage and in vivo susceptibility to MHV infection in resistant (SJL) and susceptible (B6) mice are concordant. (2) To study the rolw of virus-receptor interactions in host range specificity, we will: (2a) identify receptors in murine and human cell lines which are recognized by the host range mutant MHV-H2 and its revertants, H2$6 and H2$11, 92b) map receptor residues that bind MHV-H2 to mediate interspecies transfer into human and murine cell lines, (3c) measure MHV-H2 and revertant virus binding coefficients for soluble receptors, (4d) map alleles in MHV-H2 and revertant virus which mediate interspecies transfer into human, hamster and murine cell lines, and (5e) determine the degree of concordance between in vitro receptor usage and virulence in the original and adopted hosts. These studies will provide a basic understanding of the molecular mechanisms driving the emergence of new viral diseases in nature.